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The Center for Bright Beams, A National Science Foundation Science and Technology Center

New simulations of magnetic vortex entry at grain boundaries

Jared Carlson, Alden Pack, Mark K. Transtrum, Jaeyel Lee, David N. Seidman, Danilo B. Liarte, Nathan S. Sitaraman, Alen Senanian, Michelle M. Kelley, James P. Sethna, Tomas Arias, and Sam Posen

Grain boundary defects in Nb3Sn SRF cavities may limit the maximum magnetic field they optimally perform in. Using a finite element method, we computationally simulate these defects by mimicking surface roughness and Sn segregation. We find even small defects reduce the maximum magnetic field.

Nb3Sn SRF cavities operate at higher temperatures and theoretically higher magnetic fields than Nb. It is thought that grain boundaries may be limiting the maximum magnetic field. By simulating grain boundaries we bring to light complicated physics that can't be easily measured and may bring insight as to how to design better Nb3Sn SRF cavities.

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Superconducting film with a simulated vertical grain boundary. More in caption.

A horizontal chart of red to green sits next to a vertical color key.

Here we plot the norm squared of the order parameter for a superconducting film with a simulated vertical grain boundary in the middle. We have periodic boundaries on the left and right side and a fixed magnetic field is applied on the top and bottom of the film. The norm squared of the order parameter is a measure of how superconducting the material is with 0 being nonsuperconducting and 1 being a normal superconducting state. The magnetic field comes out of the page. The blue dots are the center of magnetic vortices that first penetrate along the grain boundary and then get pushed into the bulk superconductor. This behavior is thought to be one of the reasons current SRF cavities do not optimally perform at the theoretical maximum magnetic field. Length is measured in penetration depths.
 

Link to full publication:

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.103.024516